Processes such as embryonic development, cell differentiation or adaptation to new environments all involve changes in the levels of expression of various genes. Our understanding of these processes is, therefore, critically dependent on our knowledge of the molecular mechanisms of gene regulation. Heat shock protein (hsp) genes will be used as a model system for studying, at the molecular level, aspects of gene regulation in several of the above processes. These genes are typically expressed at very low levels but are activated at elevated temperatures and by a number of cellular stresses, and their products are thought to protect cells against stress damage. Some of these genes are also active at different stages of normal development. Heat-induced expression of Drosophila and other hsp genes appears to depend on the presence in the promoters of a regulation unit consisting (minimally) of two binding sites for a hsp gene-specific transcription factor. This factor, which is continuously present in cells, is activated by stress and then binds to the regulation unit. Earlier findings suggest that the factor molecules binding to the regulation unit need to interact to produce promoter activity. We propose (1) to continue in vivo and in vitro studies to define the relevant interactions between factor molecules binding to the heat shock unit, (2) to raise antibodies against such a factor and to obtain its cDNA gene to define the nature of the interactions between factor molecules and the mechanism of factor activation, (3) to purify and characterize another factor likely to be involved in the developmental regulation of the Drosophila hsp23 gene (ecdysterone receptor), and (4) to characterize a second type of developmental regulation of the hsp23 gene.